The AMTEC cell is a thermally regenerative electrochemical power cell used in space power systems for production of electricity from very high heat sources. It may also be used terrestrially as a remote power source, or as a vehicular (land, sea or water) power source, and at a lower scale as a co-generation power source wherever very high heat waste is present at an industrial plant. In a small scale, it is particularly useful wherever limitations on system mass or volume are severe, such as in spacecraft or satellites.
Typical heat input temperatures range from 900.degree. to 1200.degree. K. with heat rejection temperatures from 400.degree. to 800.degree. K. In the high temperature region, AMTEC technology uses a beta-alumina type solid electrolyte (BASE) plate to separate high activity sodium (Na) from a region in which the sodium activity is kept low by a condenser operating at the heat rejection temperature. BASE is a conductor of sodium ions (Na+) and an insulator for electrons.
Typical AMTEC designs use liquid Na as the anode on the high activity side [C. P. Bankston, T. Cole, S. K. Khanna and A. P. Thakoor, "Alkali Metal Thermoelectric Conversion (AMTEC) for Space Nuclear Power Systems," Space Nuclear Power Systems, 1984, Orbit Book Co., Malabar, Fla., 1985, pp. 393-402; R. K. Sievers and C. P. Bankston, "Radioisotope Powered Alkali Metal Thermoelectric Converter Design for Space Systems," Proceedings of the 23rd Intersociety Energy Conversion Engineering Conference, Vol. 3, 1988, pp. 159-167; N. Weber, J. R. Rasmussen, G. Harkins and S. L. Olsen, "Design and Performance of a Small Circulating Sodium Heat Engine," Proceedings of the 23rd Intersociety Energy Conversion Engineering Conference, The American Society of Mechanical Engineers, 1988, pp. 215-217, all of which are incorporated herein by reference]. At 1160.degree. K., the vapor pressure of Na is 1 atm while the Na vapor pressure at the condenser is 10.sup.-5 atm or less. This vapor pressure ratio results in an electrical potential across the BASE of up to 1.6 V. Electrodes on either side of the BASE provide sites for an electrochemical reaction required at the interface.
On the high activity side, the liquid Na anode provides sites for Na oxidation. On the low activity side of the BASE, the cathode (typically a thin, porous metal) provides sites for reduction of Na+. Electrical power can be drawn from the cell when the anode and cathode are connected to a load. A complete description of the AMTEC cycle has been published previously. [N. Weber, "A Thermoelectric Device Based on Beta Alumina Solid Electrolyte," Energy Conversion, 14, 1974, pp. 1-8, incorporated herein by reference.]
AMTEC cells produce electrical power at high efficiency and power density, but the output per cell is typically about 0.5 V and up to 100 A at maximum power. Power conditioning electronic circuits cannot efficiently convert this low voltage, high current power to a more useful voltage level. Previous AMTEC systems designs required connecting independent modules in series to produce a more useful voltage to the power conditioning electronic circuit. While highly redundant, that approach resulted in systems with significant mass and volume attributed to nonpower producing components.
Previous vapor-fed designs used sodium phase change to supply sodium working fluid and additional heat to the cell. Some of the sodium refluxes back to the boiler. As yet unknown is the effect of using very low oxygen content sodium on the beta alumina solid electrolyte during such a reflux process. Over time, the sodium may act as a getter to extract oxygen from the electrolyte and deposit it on the metal walls of the sodium boiler.
In order to achieve very high efficiency operation, previous AMTEC designs require electrical feedthroughs that operate at high temperature in a sodium environment. Under such conditions, the required insulator may degrade and limit the life of the cell.